Message reception apparatus



P 1939. J. E. DICKERT 2,171,625

MESSAGE RECEPTION APPARATUS Filed Jan. 18, 1938 fidelzzbrx Patented Sept. 5, 1939 UNITED STATES PATENT OFFICE 8 Claims.

The invention relates generally to the transmission, reception and reproduction of intelligence or signals by electrical means such as telephone, telegraph or radio apparatus, and

more particularly it relates to means for suppressing objectionable noise induced in such apparatus by various kinds of peak type electrical disturbances.

It is well recognized that the noises which are ill most objectionable in either short wave or broadcast reception are of relatively short duration, such for example, as those disturbances created by automobile ignition systems, and that by reason of this short duration, it is possible to suppress both the noise and the signal during the period of noise duration Without objectionably affecting the apparent continuity of the signal.

Considering the reproducer in receivers of the various types as being connected to the audio load circuit of the rectifier or detector at a particular take-01f point on such load circuit, such total suppression of both the noise and the signal has been attained heretofore by providing what amounts, in effect, to a short circuit around such point to ground, such short circuit being rendered effective automatically by and during noise peaks exceeding a predetermined value or noise threshold.

: While this short circuiting action prevents energization of the reproducer by the peak voltage during such peak type disturbances, and thus avoids the production of audible noise by such peak voltage per se, I have found that the variations of the voltage incident to the operation of such prior devices have introduced different types of disturbance into the reproducer as well as other elements of the receiver circuit, which, although not nearly as objectionable as the original noise impulses would have been, constitute audibly perceptible secondary variations from the sound which would be reproduced by the unimpaired or undisturbed signal.

The primary object, therefore, of the present invention is to provide a new and improved noise silencer'which operates to suppress noise more effectively and satisfactorily than prior silencers.

Another object of the present invention is to provide a new and improved noise suppressor for radio receivers, and the like, which during periods of suppression of noise impulses above a predetermined threshold operates to eliminate or reduce materially such secondary variations of the reproduced signal.

Another object is to provide such a suppressor operable during its periods of suppressing action to maintain the reproducer in an energized condition closely approximating its energization by the unsuppressed carrier wave.

Another object is to provide a new and im- 5 proved suppressor wherein both the threshold of noise suppression and the level of energization of the reproducer during suppression periods are automatically a function of the average value of the carrier signal strength. 10

A further object is to provide a new and improved noise suppressor having its threshold of noise suppression automatically related to the average carrier signal strength by means operable to maintain this threshold substantially un- 5 disturbed during a succession of frequently repeated noise peaks of substantial power.

Another object is to provide such a noise suppressor which is operable automatically to suppress noise peaks above a predetermined thresh- 20 old while maintaining a source of A. V. C. voltage for use in other parts of the receiver at substantially the average level which was efiective just prior to the suppression period, whereby to preserve the advantageous results of the normal 25 A. V. 0'. action while preventing any disadvantageous disturbances within the receiver clue to changes in such A. V. C. voltage induced by noise peaks above said threshold.

Other objects and advantages will become ap- 30 parent from the following description taken in connection with the accompanying drawing, in which the figure is a circuit diagram of a portion of a radio receiver constructed to embody the preferred form of the invention. 35

For purposes of disclosure the invention is i1- lustrated in the drawing and will hereinafter be described in detail as embodied in a particular form of radio receiver, and at a particular point in said receiver, but it is to be understood that 40 this disclosure is not intended as a limitation of the invention to radio receivers or to this type of receiver, or to the particular point in said receiver, it being contemplated that various changes might be made by those skilled in the art to 45 adapt the invention to other types of devices and relationships Without departing from the spirit and scope of the invention as defined in the appended claims.

In the form chosen for disclosure herein the invention is illustrated as embodied in a radio receiver of a conventional design to suppress interfering noise impulses of the short duration, high peak type, such as those interfering 55 cause of this fact the theory herein stated is not to be considered binding. In order that others may construct the present device and obtain proper functioning thereof irrespective .of the correctness or incorrectness of the theory stated,

I will also set forth approximate values and relationships of these elements which I have found to produce satisfactory operation, and will provide complete instructions as to the manner of constructing, using, testing and adjusting said device whereby a person skilled in the art may obtain optimum results. 1

In the particular circuit illustrated in the drawing, signal impulses are received from the intermediate frequency portion (not shown) "of the receiver through a'ooupling such as an impedance l0 which'in this form of coupling constitutes the secondary of an intermediate frequencytransformer l0, and the impulses are supplied to asignal rectifier II. The rectifier l l, in the form shown, comprises a diode having a plate 12 and a cathode I3, the plate being connected by a wire I4 to one end of the impedance III, while the cathode I3 is connected to ground at I 5 by wires I 6 and I6, resistance R1 and wire l6. For the sake of simplicity, the heater circuit to the diode is not shown.

At its other end the input impedance l0 has a circuit to ground, as at I5, this ground'circuit as herein shown comprising wires 20 and 2|, resistance R2, wires 24 and 25, resistance R3, and wires 26 and 21 and wire l6", all in series to ground at l5. r r

In conventional operation of a receiver of th general type herein shown, the resistance R3 constitutes the useful load resistance between the plate I2 of the signal diode andgroundyand in this particular type of conventional circuit, the resistance R3 is used in conjunction with a movable contact 30 to provide a volume control connection to a conductor 3| which transmits the power or signal to the reproducer or audio amplifier section (not shown) of the receiver.

In the circuit thus described, any outside interfering voltage impulse is normally rectified by the signal diode II and produces an increase in potential across the load resistance, :50 as to vides for rendering such interference inefl'ective' upon the reproducer when the interference exceeds the average signal strength by more than a predetermined ratio, the predetermined point thus'defined constituting what is herein termed thethreshold of noise suppression. V v I Proceeding on the known fact that peak-type 1nterferenc'e impulses such as those produced'by auto ignition systems are of extremely short duration (that is, in the order of approximately A of one second), the present invention provides means acting automatically during the noise peak to prevent both the signal and the interfering noise impulse from reaching the point at which the lead or connection to the reproducer or amplifier apparatus is taken from the circuit.

While the present embodiment of the invention provides for automatically relating the threshold of noise suppression to the average carrier signal strength, or in a predetermined ratio thereto, it is to be understood that in some of its aspects, the present invention is independent of this automatically fixed threshold.

In preventing the combined noise and signal voltage from reaching the point of take-off of the lead to the audio section, the present invention provides an automatically controlled alternative circuit for this combined voltage of such a character that the voltage effective at the point of take-off of the lead to the audio section is, during the noise period, off-set by a substantially equal voltage of opposite polarity. In addition the present invention provides means whereby said point of take-off of the lead to the audio section is, during the suppression periods, maintained at the same polarity and at substantially the same average potential as it had just prior to the suppression period.

Through this last mentioned action a relatively constant A. V. C. source is maintained substantially constant at its just previously determined level during suppression periods, thereby rendering the A. V. C. means operable in a uniform manner even during continued noise peaks. Hence the present suppressor cooperates with the normal A. V. C. means in relating the A. V. C. eifectiveness at all times solely to the average carrier level rather than to the average carrier-plus-noise level.

The alternative noise dissipating circuit of the present invention comprises fundamentally a connection from the audio take-off side of the detector to a source of voltage which is of opposite polarity but varies with the signal or signal and noise level. In its present form this alternative noise dissipating circuit comprises a connection between the juncture 34' of wires 2:3 and 25, and the circuit formed by wires l6 and 16', R1 and wire [6, such connection being normally non-conductive, and being rendered conductive during noise peaks by an automatic control device 33. Thus, as shown in the figure, a wire 34 extends from the juncture 34 to one side or terminal of the control device 33, while a wire 35 extends from the other side or terminal of said device 33 to a juncture 35 with a shiftable' contact arm 36 associated with the resistance R1.

Hence, when the automatic control device 33 provides for conduction of current between the wires 34 and 35, there is a circuit formed through the signal diode H independent of ground. Moreover, the wire 35, being connected at 35' through wires l6 and. IE to the cathode I3 of the signal diode, has a positive potential, that is, in the present case, a potential opposite to the potential of the juncture 34. Preferably this point 35' is chosen with a definite voltage relation to the point 34', the present embodiment being such as to provide for relative adjustment of the voltage at 35' by means of the variable connecting member 36 interposed between wire 35 and the resistance R1. Thus in the form shown, with all or a substantial portion of the resistance R1 effective between the point 35 and ground, the point 35 normally has a positive potential of a slightly greater value than the negative potential at point 34, the required excess value of voltage at 35 above the value of the voltage at 34 being determined, in part, by the impedance of the control device 33 when conductive.

In the form herein shown the control device 33 is essentially a rectifier, and is preferably in the nature of a grid controlled rectifier of the thermionic type, so that the device may be so connected as to provide for an automatically adjusted threshold Value under the governing control of the average signal strength. To this end the wire 36 is connected to cathode 36 of the rectifier 33 while its, plate 3'! is connected to the wire 35. The rectifier 33 employed in the present instance has a control grid 38, and the tube is of the zero bias type wherein the tube is essentially non-conductive so long as the grid is more negative than the cathode. The cathode 36, as herein connected, has a negative potential with varies with the rectified output of the signal diode; and to provide the desired automatic threshold action of the control device, the grid 38 is connected through a voltage averaging and time delay means to a point having a relatively higher negative voltage varying with said rectified output.

To this end the grid 38 is connected by wires 39 and 40, resistance R4 and wire 4| to the juncture of wires 39 and 43, a capacity is provided to ground by wire 42, capacitance C1 and wires 43 and M to ground at 45. With this arrangement, if the resistances R2 and R3 are equal, the point to which the wire 4| is connected has twice the negative potential of the point 34; and if the resistance R4 and the capacitance C1 have values sufficient to prevent change of the grid voltage during the time duration of a noise peak, such peak voltage will render the cathode 36 more negative than the grid 38 so as to render the tube 33 conductive during such peak impulse. With the resistances R2 and R3 equal, the thresh old of noise suppression will be at substantially twice the average carrier level, whereby to provide for 100% modulation of the carrier. By reducing the value of R2 with relation to the value of R3, this threshold may be lowered as desired.

Since the described circuit is such that the point 35' may, as determined by the adjusted or effective value of the resistance R1, have a positive charge equal to or slightly exceeding the negative charge at the point 34, the conductive relation between the cathode 36 and plate 31 of the tube 33 during noise peaks causes the combined signal and noise voltage to be offset and ineffective at the point 34'. Hence any interfering voltage above the automatically established threshold is rendered ineffective for the production of audio or A. V. C. voltages, and since this oilsetting action does not involve or require flow of current into the line 39 to the condenser C1, frequently repeated noise impulses of relatively great power may be dissipated by thepresent device without building up orincreasing the threshold-establishing charge of the condenser C1. Since the bias of the tube 33 during noise peaks is such as to establish a conductive relation between the grid 38 and cathode 36, the stored negative power charge on the condenser C1 is, in effect, connected to the point 34', so that while the combined noise and signal voltage at point 34 is, during a noise peak, offset by the cathode-plate conductivity of tube 33, the negative charge on the condenser C1 which is at the average carrier voltage, is applied to the point 34' during such peaks. As before stated, the positive voltage at 35' exceeds slightly the value required to offset the negative component of the rectified output of the detector and this excess is soproportioned by adjustment of the effective value of R1 by the element 36, as to offset a portion of the voltage applied to the point 34 from the condenser C1, whereby to charge the point 34 to a value closely approximating the charge at that point just prior to the noise peak. The leakage from the condenser C1 to point 34 does not, in most instances, reduce the charge of the condenser C1 to any appreciable extent, because this condenser is of relatively large capacity, but if any reduction in the charge of the condenser becomes apparent in any particular circuit design, the rate of leakage may be controlled by a suitable resistor interposed in the wire 39.

In addition, there may be a slight amount of grid current tending to increase the charge of the condenser C1 but in the main, the negative rectified output of the detector is dissipated as plate current in the control device 33 rather than as grid current, and as will hereinafter be pointed out, the present device may be so adjusted and proportioned that during a series of noise peaks of substantial power the voltage on the grid or lead wire 33 will remain substantially constant at the previously determined average value.

Thus, with the present suppressor, the juncture 34' provides a point which assumes an average value for a given carrier; and which maintains substantially that same average value even when noise is added to the carrier; and it will therefore be apparent that the A. V. C. connection 50 taken from the point 34 provides for A. V. C. which operates uniformly to maintain the desired sensitivity in the circuit as a whole. Hence the A. V. C. actually cooperates with the present suppressor circuit in increasing the intelligibility of the received signal.

In constructing the present device, I prefer to employ a threshold of noise suppression which allows for 100% modulation of the carrier, and hence the resistances R2 and R3 are made of substantially equal value, the resistance R3 being of a value proportioned to the other elements of the receiver in accordance with common practice. For example, I have, in an operating embodiment of the invention, used a value of 100,000 ohms for the resistance R3. As a control device 33, I have found that a 6A6 tube operates satisfactorily, and is associated with a time delay and voltage averaging means having a time constant of approximately of one second. In actual use I have found that by making R4. of one megohm resistance, and the condenser C1 of one tenth microfarad capacity, satisfactory results are obtained. The values of R4 and C1 may of course be varied so long as a time constant of suificient magnitude is maintained, a sufficiently conductive by pass for alternating components is provided through C1 to ground and a high enough resistance is maintained in R4 so as not to unduly load the signal diode.

The resistance R1 is preferably of a value which exceeds by a considerable amount the value of R3 (but variable in its effectiveness upon the voltage at 34'), say twice'the value of R3; and to adjust the device for optimum performance, a suitable voltage measuring device is connected to the juncture of wires 40 and 42. A source of rapidly repeated peak type electric discharges is then brought into play, as by means of an idling automobile engine, so as to provide a series of high peak impulses which will be superimposed upon or added to the carrier wave. When such interference is stopped, the voltage at point 40, 42 will assume an average value, determined by the carrier wave, while starting of the interference-creating means will produce a perceptible variation in such voltage. By varying the position of the member 36', the amount of this variation of voltage may be adjusted, and it will be found that in one position of the arm 36', there will be no practical variation of the voltage reading after the interference-creating means is started. In this position of adjustment optimum performance of the present device will be obtained.

I claim as my invention:

1. In a carrier wave receiver, the combination of a circuit carrying desired pulsating unidirectional current of a given polarity, and. also having undesired transient currents flowing therein, said desired currents having an average value at a given point in said circuit dependent on the intensity of the received carrier'wave, and an instantaneous value depending upon the desired modulations of the carrier Wave, an additional circuit including a grid controlled thermionic vacuum tube rectifier having its cathode connected to said point in said first mentioned circuit and a source of pulsating unidirectional current of opposite polarity varying with said desired modulation and said undesired currents, said last mentioned source being connected to the plate of said rectifier, and means connected to the grid of said rectifier and. operable to supply a threshold establishing bias for said rectifier.

' 2. A carrier wave receiver having in combination with a first conductor having first and second sections consisting of impedances across which appear voltages of given polarity, said voltages being supplied by desired currents and by undesired transient currents of high voltage flowing in said first conductor, a useful voltage tap associated with said second section, and means for rendering the high voltages of said undesired transient currents ineffective at said useful voltage tap, and means comprising a second conductor consisting of a tapped impedance across which appears a voltage of opposite polarity and varying in opposite ratio to voltages across said first conductor, a zero-bias grid controlled thermionic rectifier having its cathode connected between the sections of said first conductor, means connected to said grid operable to supply an average voltage having a predetermined relation to the average value of said desired currents whereby to establish an operational threshold for said tube, and. means to supply the plate of said tube with voltage from said tap on said second conductor.

3. A carrier wave receiver having in combination with a first conductor having first and. second sections consisting of impedances across which appear voltages of given polarity, said voltages resulting from desired currents and from undesired currents flowing in said first conductor, said desired currents having an average value depending on the strength of the received carrier wave and an instantaneous value depending on the desired modulations of the carrier wave, a useful voltage tap associated with said second section, and means for rendering said undesired currents ineffective at said voltage tap when the voltage of said undesired currents exceeds said average value by a predetermined ratio, said means comprising a second conductor consisting of a tapped impedance across which appears a voltage of opposite polarity and varying in opposite ratio to voltages across said first conductor, a zero-bias grid controlled thermionic rectifier having its cathode connected between the sections of said first conductor, time delay and voltage averaging means connected between said first section of said first conductor and said grid to charge said grid with a related, threshold-establishing voltage for said tube, and means to supply the plate of said tube with voltage from said tap on said second conductor.

4. In combination with a first conductor having first and second sections consisting of impedances across which appear voltages of given polarity, said voltages resulting from desired currents and from undesired currents flowing in said first conductor, said desired currents having an average value depending on the strength of the received carrier wave and an instantaneous value depending on the desired modulations of the carrier wave, a useful voltage tap associated with said second section, and means for rendering said undesired currents ineffective at said voltage tap when the voltage of said undesired currents exceeds said average value by a predetermined ratio, said means comprising a second conductor including impedance means across which appears a voltage of opposite polarity and varying in opposite ratio to voltages across said first conductor, a zero-bias grid controlled thermionic rectifier having'its cathode connected between the sections of 'said first conductor, time delay and voltage averaging means connected between said ficrst section of said first conductor and said grid to charge said grid with a related, thresholdestablishing voltage for said tube, and means to supply the plate of said tube with voltage from said second conductor bearing a predetermined relation to the voltage appearing on said first conductor.

5. A carrier wave receiver having in combination with a first conductor having first and second sections consisting of impedances across which appear voltages of given polarity, a useful voltage tap associated with said second section, a second conductor across which appears a voltage of opposite polarity and varying in opposite ratio to voltages across said first conductor, a zerobias grid controlled thermionic rectifier having its cathode connected between the sections of said first conductor, means connected from said first section to said grid and operable to supply a related average voltage whereby to establish an operational threshold for said tube, said last mentioned means including time delay and voltage averaging means, and means to supply the plate of said tube with voltage from said second conductor.

6. In a carrier wave receiver, the combination of a first circuit carrying desired pulsating unidirectional current of a given polarity, and also having undesired transient currents fiowing therein, said desired currents having an average value depending upon the intensity of the received carrier wave and having an instantaneous value depending upon the desired modulations of the received carrier wave, an audio voltage lead tapped into said circuit, a second circuit carrying pulsating unidirectional current of opposite polarity varying with said desired modulations and undesired currents, and an additional circuit to prevent such undesired currents of the peaktype from reaching the tap of said audio voltage lead, said additional circuit comprising a zerobias thermionic rectifier tube having its cathode connected to the one of said circuits carrying current of negative polarity and its anode connected to the one of said circuits carrying current of positive polarity, and storage means comprising a resistance and capacitance connected to the one of said circuits carrying current of negative polarity and connected to the control grid of said tube to provide an operational threshold for said tube controlled by and varying automatically in proportion to said average value of the received carrier wave.

7. In a carrier Wave receiver, the combination of a demodulator having a first circuit from one of its elements carrying desired pulsating unidirectional current of negative polarity and also having undesired transient currents flowing therein, said desired currents having an average value depending upon the intensity of the received carrier Wave and having an instantaneous value depending upon the desired modulations of the received carrier wave, a second circuit from another element of said demodulator carrying pulsating unidirectional current of positive polarity varying in opposite ratio to the currents in said first circuit, a zero-bias thermionic rectifier tube having its control grid connected through time delay and voltage averaging means to a first point on said first circuit, means connecting the cathode of said tube to said first circuit at a second point having a voltage less than the voltage at said first point, means connecting the anode of said tube to said second circuit, and an audio voltage tap on one of said circuits at a third point such that the point of connection of the anode-cathode circuit of said tube with said one of the circuits is located between said demodulator and said third point.

8. A carrier wave receiver as defined in claim 7 wherein said audio voltage tap is made on said first circuit.

JAMES E. DICKERT. 

